1. Field of Invention
At least one embodiment of the present invention relates generally to an assembly and method for securing a hub to a shaft, and more specifically, to a hub-shaft assembly incorporating axially aligned fastening devices to secure the hub to the shaft.
2. Discussion of Related Art
A common method for locking hubs of various mechanical components (e.g., gears, brake wheels, sprocket wheels, shaft couplings, cranks, cams, etc.) to shafts for the purpose of transmitting torque or rotary motion is by use of a key installed between the shaft and hub. The key cross section is generally square or rectangular (e.g., parallel or tapered). These keys lock the hub to the shaft against relative rotational movement, but the relative axial movement is restrained only by friction between the hub, shaft, and key. Positive restraint can be provided, but only in one direction, by an interlocking shoulder on the shaft or hub.
The recommended sizes and dimensional tolerances for parallel and tapered key assemblies, based on shaft diameter and class of fit, are tabulated in USA Standard USAS B17.1-1967, Keys and Keyseats.
These key installations have several inherent shortcomings:
Because the keyseat in the shaft and the keyway in the hub are machined independently of each other, the machining and indexing require a high degree of accuracy for a proper match with each other and with the key. The possible mismatches and/or misalignments, that degrade the fit (e.g., the desired and/or required bearing or amount of metal-to-metal contact) of the key installation and reliability of the hub-shaft connection, are also depicted in this standard. There is no reliable way of determining the extent of these mismatches and/or misalignments, and the resulting degradation of the torque-carrying capacity, after the installation. Poor bearing contact, such as on high spots or along edges, raises the localized bearing stresses and leads to surface deformations, loosening of key, and/or failure of the connection.
The 90-degree corners of the keyseat, even when filleted, cause stress concentrations that degrade the strength of the shaft. The above industry standard presents suggested fillet radii and the corresponding key chamfers “as a guide.” According to the authoritative Peterson's Stress Concentration Factors, Second Edition, Chart 5.2, by Walter D. Pilkey, the stress concentration factors (stress increases due to this geometric discontinuity) for the suggested fillet radii vary between 2.4 and 2.8. But USAS B17.1-1967, Table 7 states: “In general practice, chamfered keys and filleted keyseats are not used. However, it is recognized that fillets in keyseats decrease stress concentrations in corners. When used, fillet radii should be as large as possible without causing excessive bearing stresses due to reduced contact area between the key and its mating parts.” In practice, it is customary to specify a 0.005-inch radius for the corner fillets. For this fillet radius, the stress concentration factor (stress increase due to this geometric discontinuity) is 4.00 according to Peterson's Stress Concentration Factors. 
The stress concentrations in the keyseat (and keyway) corners can become sites for initiation of fatigue stress cracks. Metal fatigue is the progressive (often initiated at a point of stress concentration) structural damage that occurs when a material is subjected to cyclic loading. The actual stresses are below the ultimate tensile stress limit, and may be less than the yield stress limit of the material.
American National Standard ANSI/AGMA 6001-D97, Design and Selection of Components for Enclosed Gear Drives, is the authoritative industry standard for the design of gearbox shafts (and other gearbox components). It provides graphic and quantitative analyses of the various features and conditions that degrade the fatigue strength of shafts. Although this standard was developed specifically for the enclosed gear drives, its shaft design methodology is applicable to the design of shafts in general.
In some key installations, a single key is not adequate for the torque to be transmitted. It is therefore necessary to install a second key. Frequently, the amount of indexing error requires the second key to be a “hand-made stepped” key. See, e.g., FIG. 1, which shows the shape of a hand-made stepped key required in cases where the second keyseat and keyway are not indexed precisely from the first set. (The offset/step is shown exaggerated for clarity.) Furthermore, for solid metal-to-metal contact, radial and axial cant may be required on the side faces of the stepped key. (The top and bottom faces need not make contact with the hub or shaft.)